class Solution {
public:
    int numTrees(int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
      if (n == 0) {
        return 0;
      }
      vector<int> dp(n + 1);
      dp[0] = 1;
      dp[1] = 1;
      for (int i = 2; i <= n; ++i) {
        for (int j = 1; j <= i; ++j) {
          dp[i] += dp[j - 1] * dp[i - j];
        }
      }
      return dp[n];
    }
};
